Your shock use multiple seals to protect parts and seal assemblies from each others and the outside. Seals are not eternal and require replacement on a regular basis, outside and inside.
4. New internal wear parts
MTB shock are not just a flexible part: there is multiple parts inside, including wear parts that require proper care.
Wear parts are parts that will get worn out with normal use and should not be mistaken with broken parts. Beyond seals, there is also sealheads, guides and bumpers. Internal wear parts can only be replaced after a disassembly of the shock.
5. Disassembly, control and rebuild
MTB rear shocks are assemblies of mechanical parts that are threaded and pressed with each others. Assemblies will disassemble when the shock is used.
After 100h of use, full disassembly, check of every part and rebuild is the way to make sure your rear shock remain in good shape and safe to ride.
6. Expertise and equipment
Professional servicing requires specialized knowledge, tools, and expertise to thoroughly inspect and maintain fork internals, ensuring comprehensive care and issue identification.
More importantly: most rear shocks are difficult to replace
Fork are bolted on bikes using a steering tube so that the front is quite independent from the rest of the bike. That is not the case for rear shocks. Rear shocks are installed in the middle of the frame and almost always have specifications for that frame: size, travel, hydraulic settings, specific springs - a combination which is hard (and expensive) to replace. Proper service prevent the need to replace the rear shock.
Recommended Rear Shock Servicing Intervals for Major Brands:
Fox: Fox recommend full shock service every 100h or annually whichever comes first
Rockshox: Rockshox suggest full shock service every 50, 100 or 200h depending on the model and parts; in practice it is best to do it every 100h, otherwise the next 100h will be less than optimal
Öhlins: Öhlins suggest full shock service every 100h of riding
